Noise figure (NF) is a widely-used metric for describing signal-to-noise-ratio (SNR) degradation that occurs to a signal passing through an electrical network. The noise factor (F) of a network is generally defined as the input SNR divided by the output SNR: F=(Si/Ni)/(S0/N0), where Si=input signal power, S0=output signal power, Ni=input noise power, and N0=output noise power. NF is the noise factor expressed in decibels: NF=10*log (F).
For non-passive electronic components, a significant source of degradation is noise generated from internal active devices, such as transistors. Accordingly, NF measurements are often a necessary part of device characterization in research and development, and process verification in manufacturing.
Two techniques are commonly used to measure NF: a Y-factor method and a cold-source method. The Y-factor method (also called hot/cold-source method) is the predominant approach and is most commonly implemented with noise-figure analyzers and spectrum analyzer-based solutions. The Y-factor method uses a calibrated noise source that includes a noise-optimized avalanche diode that can be turned on and off. The diode is followed by an attenuator, which ensures a good output match.
Where the diode has no bias applied (i.e., in a cold state), the noise source generates the same noise as a room-temperature termination. Where bias is applied to the diode (i.e., in a hot state), a resulting avalanche breakdown creates considerable electrical noise over and above that of a room-temperature termination. The amount of extra noise is characterized as an “excess noise ratio” (ENR). Typical ENR values are in the range of 5 to 15 dB. The gain and noise figure of a device-under-test (DUT) can be determined from two separate measurements of output-noise power resulting from the cold and hot input terminations. Assuming calibrated noise-power measurements, noise factor F=ENR/(Y−1), where Y=No-hot/No-cold, i.e., a ratio of DUT output noise powers in the hot and cold states of the noise source at the DUT input.
In contrast, the cold-source method is usually performed using a vector network analyzer (VNA), which provides magnitude and phase information, making it possible to achieve greater measurement accuracy by using advanced error-correction methods. The improved accuracy may be most dramatic for non-coaxial environments such as those where the DUT is measured in a fixture or while still part of a semiconductor wafer. Due to the improved accuracy, the cold-source method is preferred in many component-test scenarios.
The cold-source method combines traditional S-parameter measurements of a DUT with a single measurement of output-noise power resulting from a cold input termination (typically at room temperature). These two portions of the NF measurement generally happen sequentially, as follows. First the DUT's S-parameters are measured using a built-in sinusoidal source and standard VNA receivers, and the DUT's gain is determined from the S-parameters. Second, the sinusoidal source is turned off, and an output-noise power measurement is made using either a dedicated low-noise receiver or one of the standard VNA receivers. An underlying principle of the cold source method can be appreciated by rearranging the terms in the above definition of NF and substituting gain (G) for the ratio S0/Si:F=No/(G×Ni). For a known input noise (Ni) due to an input termination at a known temperature, F can be calculated by measuring the gain (G) and output noise (No) of the DUT.
Many devices exhibit a gain and NF response that has relatively small variation versus frequency. For such devices, the Y-factor and cold-source method produce substantially the same results, with varying degrees of accuracy. Many other devices, however, contain integral bandpass filters, which result in large changes in gain and NF at transition regions between the filters' in-band and out-of-band response, the cold source method may introduce distortion in the noise figure measurement which does not occur with the Y-factor method. If the DUT's bandwidth is not significantly larger than the bandwidth of the receiver used to measure the output noise, the distortion can be significant.
In view of these and other shortcomings of conventional approaches, there is a general need for new techniques for making NF measurements.